Improving clay brightness by flotation and fine grinding



brightness tester.

United States Patent IMPROVING CLAIY BRIGHTNESS BY FLOTATION AND FINEGRINDING James B.-Duke,fMetwchen, N.J., assignor to Minerals & ChemicalsCorporation of America, Menlo Park, N.J., a corporation of Maryland NoDrawing. Application December 16, 1957 Serial No. 702,820

13 Claims. (Cl. 241-23) This invention relates toa process forconcomitantly improving the brightness .and attaining particles ofextremelyminute particle sizes from relatively coarse and discoloredclayby'a novel sequence of steps. Pursuant to the invention acoarse fractionof kaolin clay, discolored by what is considered to be essentiallytitaniferous material, is subjectedto a combination of steps whichcooperate to produce a product 'having qualities for use as apaper-coating pigment comparable to those of certain high-grade importedsedimentary clays which are distinguished for their whiteness andfineness and are superior to the highest grade domestic kaolin claysprocessed by prior art methods. Such a coarse fraction of kaolin clay isobtained during the classification of kaolin clay and is consideredto'be a waste fraction thereof. The terms kaolin" or kaolin clay as usedherein indianite. These minerals are'hydrous aluminum silicatesgenerally represented by the formula n usually being 2. The weight ratioof silica to alumina -is about 1.0-1.5 to '1, and most generally about.l.l8 to '1.

The use of kaolin clays in the production of high-grade paper coatingshaslbeen restricted to date to .those expensive finely-particulatedkaolin clays having 1 high brightness asmeasured by the standard General"Electric Such clays may be obtained from certain deposits ofsedimentary clay found'abroad, .or'in .the case of domestic sources byfractionating relatively bright primary clay torseparate therefrom'afine .fraction suitable after an additional bleaching operationfor usein paper coatings. The suggestion has been made in US. Patent No.2,569,680 to T. GuLeek to subject a clay, particularly a finesedimentary clay dark because of the accretion of certain 'ferruginousand titaniferous matter, to a froth flotation agent-in the presence'ofcertain negative and positive-ionreagents to float off such impuritiesand to deliver in the .tailings a clay relatively free from impurities.Considerable brightening results from such a process, particularly whensuch a process is used in conjunction with both a preliminarysedimentation or settling treatment whereby relatively large particlesof coloring matter are removed. Brightness is further improved by ableaching operation, preferably following thefroth flotation, in whichthe clay is treated withzinc dust and sulfur dioxide gas or zinchydrosulfite, washed, filtered and dried. Although by the practicestaught by Leek the brightness of a given clay may be increasedconsiderably, by as much as 5% or more,.nevertheless the product doesnot have adequate brightness for some high-grade paper coating use whenthe clay prior to treatment has a very low brightness. It 'would beexpected that improved brightening of the beneficiated clay would result'by finegrinding the clay prior to the froth yentiomto'yprovide ,a noveland economical method of "particle size is greater than about 2 microns.

ever,';that this, does not happen. -On-the contrary theflotationoperation loses much .ofits efiiciency whenapplied to .aclay-containing a substantial fraction of particles smaller:than about3.5 microns. 1 Hence the practices taught ;;by..-Leek fail to teach amethod for upgrading coarse colored clay fractions to the fineness andbrightness :required ofykaolin forpaper coating use.

The instant inventionis the resultof .my discovery that .the brightnessof acoarse kaolin clayhighly colored by .tita-niferous (probably:anatase) or titaniferous and feruginous .matter may be brightened toheretofore unattainable brightness while simultaneously reducing thatclay to a controlled micron "particle size within a selected range :by:subjecting. a dispersion .:of :the coarse .clay 'to a froth flotationprocess in the presence of an anionic collector whereby a substantialportion of .the' colored impurities are selectively floated from theclay and the beneficiated clay from the flotation is subjected togrinding of .a character such that disintegration results from theattrition of clay particles and in some cases attrition plu-s impact.The invention provides an economical means of enhancing thevalueof-unmarketable coarse tkaolinwclay-to-fine white clay suitable foruseas apaper coating pigment, in textile treatment, cosmetics or the like.

Theclay treatment of'the subject invention can be used in lieuof achemical'bleaching step, or, more preferably, ;ble,aching may be usedasan adjunctto thetreatment ;i n-.which easeeven rfurtherimprovement inbrightness ofithefinalproductis achieved.

Accordingly it iswa principal object of the instant inbrig hteningcoarse ,kaolinclays :to a greater extent than heretofore practieed.

It is another important object to providea method-tc brighten kaolinclayand concomitantly reduce the particle ;si-ze-of the kaolin to acontrolled ,size within a selected range ofparticl sizes.

.ticulated clay displaying superior brightness can be produced from acoarse darkened essentially useless fraction of clay by subjecting :thatcoarse clay to a flotation in the presence of a flotation agentselective to the impurities'inflthe clay whereby color-bodied impuritiesare mechanically separated from said coarse clay to yield a beneficiatedclay and subjecting the thus beneficiated clay to fine-grinding ofsucha-character that the beneficiated clay particles are reduced inparticle size by attrition,

preferably by subjectingsaid particles while dispersed in afiuid streamto high velocity and turbulence. The sequence of steps above recited iscritical in the practice of my invention, i.e., the outstanding clayproduct is not achieved by grinding the clay to the desired particlesize prior to flotation. I have found that the specific flotationoperation within the scope of the invention is most beneficially appliedto a kaolin clay in which the average For example, when the clay feedcontains more than about 30 percent of minus-Z-micron (equivalentspherical diameter) particles and more than about 15 percent by weightof minus-l-micron particles the outstanding increase in brightness 'isnot achieved. Hence beneficiating a-pre viously finely-ground materialis not the equivalent in terms of ultimate brightness attainedassubjecting a beneficiated relatively coarse feed to fine-grinding ofthe character hereinafter described. Ihave found that the feed to thefine-grinding apparatus of the preferred character -is preferably arelatively coarse material, preferably a relativelyccoarse particlesize'of kaolin with' an average 3 particle diameter less than about 44microns and in the neighborhood of 2 to microns (as measured by wellknown soil hydrometer techniques).

The preferred froth flotation method, from the standpoint of performanceand economy, is that in which the coarse dispersed clay feed is treatedwith negative-ion reagents and preferably in the presence of anauxiliary agent from the group of soluble sulfates consisting ofammonium sulfate, magnesium sulfate and potassium sulfate, other solublesulfates not giving the best results. The amount of auxiliary agent usedis not critical, it havingbeen found that from about 0.5 to 10 poundsper ton (dry feed basis) is usually sufficient. Any wellknownnegative-ion collector such as a fatty acid, or mixtures containing afatty acid salt may be used. Suitable negative-ion reagents include talloil (oleic acid product or vegetable origin usually associated withresin acids), red oil (oleic acid of animal origin), linoleic acid orsulpho-oleic-acid. The flotation operation should be carried out on analkaline pulp, preferably adjusted to a pH of from about 8 to 10. The pHof the pulp should preferably be adjusted by a hydroxide of the cationof the particular sulfate employed. For example, ammonium hydroxide isthe preferred alkaline agent when ammonium sulfate is the auxiliaryagent, etc. However, in the case where magnesium sulfate is used as theauxiliary agent the corresponding hydroxide cannot be employed becauseof its relatively poor solubility in water and hence some other alkalinematerial, as forexample sodium hydroxide, should be used. The feed isordinarily pulped with water to a concentration of from about 5 to 25percent solids, and preferably 10 to percent solids.

In order to obtain the desired results it is necessary to disperse theclay prior to beneficiation with the flotanon reagents, the dispersionbeing accomplished with dispersing agents well-known to those skilled inthe art, as for example, sodium silicate, sodium carbonate, etc.

Although the preferred collector is a negative-ion reagent it will beunderstood that it is within the scope of the invention to use as thecollector an emulsion containing both negative-ion and positive-ionreagents, as taught by Leek in US. Patent No. 2,569,680. The requirementof the collector is that it be of such a charactor that the impuritiesbe selectively floated. The processing difliculties inherent in the useof a cationic collector mitigate against its use as the sole collectorin the process of the invention.

The clay is preferably bleached in addition to treatment by flotationand disintegration. The bleaching may accomplishing this pulverizationis by forcing a slurry of the beneficiated clay'in a vaporizable liquidinto and through an elongated tubular heating zone, vaporizing at leasta portion of the liquid component of the slurry during passage throughthe heating zone to form a flowing stream of a dispersion of coarse clayparticles in the resulting vapor, and subjecting the dispersion to highvelocity flow and turbulence to etfect disintegration of the coarseparticles. The process is described in detail in US. Patent No.2,735,787 to Du Bois Eastman et al. In accordance with the practicestaught therein coarse granular particles are first suspended in avaporizable liquid to form a free-flowing slurry, the termfree-flowingslurry designating a suspension of solid particles in liquidin such proportions that the suspension exhibits the general flowproperties of a liquid in that it will flow through a downwardlyextending conduit under the influence of gravity. The flowable mixtureis continuously fed as a confined stream into a heating zone in theinitial portion of a heated tube of substantial length com- 4 pared toits inside diameter. In the tube the liquid component of the slurry isheated, to a temperature well above its boiling point at the pressureexisting in the tube, thereby forming a dispersion of solids in vaporwhich flows turbulently at high velocity through the latter portion ofthe tube to accomplish the grinding. The tube is suificiently long toprovide for the vaporization of the liquid and to provide a subsequentsection of sufficient length to provide for the grinding of solids ofthe gasiform dispersion when passing therethrough. The slurry ofbeneficiated clay is, in an aspect of this embodiment of the invention,the aqueous slurry of machine-discharge clay from the flotationapparatus with the solid contents readjusted when necessary tothe-highest percentage by weight consistent with smooth function of thegrinding process. Heating requirements and costs are thus reduced. Adeflocculating agent may be added to the slurry and be distributedtherethrough to'lower the viscosity thereof. Examples of deflocculatingagents which may be used are tetrasodium pyrophosphate, disodiumdihydrogen pyrophosphate and sodium hexamethaphosphate. Suitable solidscontent of the slurry will be about 15-40 percent by weight.

Although, as above mentioned, in the particular process water is thepreferred vaporizable liquid, volatile organic liquids may be used aloneor where the organic liquid is water-miscible in admixture with water.

The process for fine-grinding a fluid dispersion of beneficiated clay issusceptible to numerous variations without departing from the spirit andscope of the invention. For example, clay particles pulverized in the-manner above described may be further disintegrated by impinging thesaid pulverized clay particles while still in the form of discretedispersed particles in high velocity flow onto plates or discs. Inanother embodiment the liquid slurry is heated to a high temperaturewhile mainthe method being treated in US. Patent No. 2,763,434

to D. M. Strasser. The dispersion thus produced is sub jected to highvelocity and turbulence to effect pulverization. The process may bemodified by passing the dispersion of pulverized clay to a fluid energymill. The fluid in which disintegrated particles are entrained, may becondensed upon exit from the pulverizer and separated from the clay or,when the vaporizable liquid is water, it may be condensed and collectedwith the attrited clay.

A preferred method of executing the discharge is to discharge the streamof dispersed pulverized clay particles into a stream of water wherebythe resultant slurry may be subjected to wet-clasification, such as forexample centrifugation. Another method of pulverizing the beneficiatedclay is by impingement of a jet of gas in which particles of the coarseclay are suspended against either a second such stream or a jet of highvelocity gas to develop turbulence therein with resultant disintegrationof particles. This gasiform dispersion is preferably made by vaporizingthe liquid component of the pulp of beneficiated clay in an elongatedzone of restricted crosssection.

Conventional fluid energy milling wherein solid particles are directlyentrained in a flowing stream of gas and circulated through an enclosedsystem confers some size reduction and additional brightness to thebeneficiated clay although the results do not measure up to thoseobtained when the gasiform dispersion of beneficiated clay is subject toturbulence at high velocity. Likewise, some enhancement in particle sizeand brightness of the beneficiated coarse clay results fromrecirculating a stream of particles dispersed in a fluid through anapparatus including serpentine and linear passages to providealternating passages of fluid and turbulent flow. Such an apbodiments ofmy invention.

eluded for illustrative.purposesponly ;and .are not to be tionings andflotations.

-.paratns ,and method is described vin UiS. .Patcnt 1N0. ,2,792 ,1l4 tKidwell etal. A

Following. are examples which gillustrate.v certain .ern- .Theseexamples .areqinconstrued as limiting the invention .to theparticular.Inodifications of said. invention disclosed therein.

EXAMPLE This example illustrates the improvement in .thebrightness ofkaolin which has been ,treated by the practice of the instantinventionover kaolin which has .been brightened by onlyflotationUand/or.bleaching. The example further illustratesthattheattainment..ofiparticles of extremely minute sizes vand within,aselectedrange ,is concomitant to enhancementof brightness.

vA coarse kaolin clay was ,beneficiated and the benefiiciated productwas finely-ground to obtainaproduct having a bleached brightness ,indexof about 90.5%.

Fractionation of this product-Produced ,afine fraction, that is one inwhich the particles were substantially ,all .of minus 3.0 micron,having. a bleachedbnightness of 92.5%, a brightness comparable to thehighest. grade imported clay. Briefly, the treatment comprisedsubjecting :the coarse discolored clay to froth flotation,andsu-bjecting the improved product therefrom to fine-grinding bysubjectingsaid product as a dispersionin Water vapor to turbulence. Theproduct thusreduced :in particle size vwas dispersed in water usingtetrasodium. pyrophosphate and fractionated by sedimentation into aminus-3 micron fraction and a plus-3 micron fraction, the coarsefraction being separately recycled throughthe reduction apparatus andagain fractionated.

A 2500 gram sample (dry weight basis).ofarrelatively coarse fraction ofa dispersed water-washed Georgia,

kaolin (in the form of a 24.4 percent solids slip) was pulped with waterinto a SOOO-gram laboratory =Minerals Separation Airflow FlotationMachine to .,a,concentration of about 10 percent solids. The kaolinofthisexample comprisedparticlesof suchsize distribution that about 44percent by weight were coarserthan7 micronsin equivalent sphericaldiameter and about 15 percent by tioned without aeration for minutesafter adding an additional 1 pound of ammonium hydroxideand- 05 pound oftall oil, after which air was admitted and a second froth productremoved for minutes. The pH of the pulp was maintained at 9 throughoutthe condi- The final machine discharge was a brightened kaolin productconsiderably lower in TiO content than the originalfeed as can beseenfrom the following metallurgical results (Table I).

Table I VOLATILE FREE BASIS The flotation machine discharge(beneficiated kaolin) was adjusted to a 25% solids aqueous slip andsubjected The j 1 to. fine..grindingbycontinuously,pumpingthe slip atrates ,{.of.7,.to 10.,ga 10'n's per hour throughpu -elongated pressuretubeheated throll houtits lengthlo maintain a discharge of kaolinparticles suspended in superheated steam ranging in temperaturelfrom,840} F. 10.1010" F. at pressures of 200 to,.15.5,.0 -pounds, ,p.ersquare inch gauge. It will be recogn zedthatthese conditions were wellunder the. critical pressure.of, 3221 p.s.i.g. .andabove the criticaltemperature .of,7,05hF.,for water vapor, thus assuring the presence ofdry superheated steam at high turbulent velocities in the: tube. Thepressure tube was discharged into cooled water thus etfecting acondensation of the steam and the recovery of the ground ordisintegratedkaolin as a suspension in water. Particle size distributionanalysis :were carried out on dispersed aqueous suspensions of the feedto and the product from this operation using well known 'hydromctersettling techniques. The results of these :analyses are listed in thefirst three columns .of Table II, and show the pronounced particle sizereduction which was obtained.

The aqueous suspension of the product from the described primaryrun wasseparated into coarse and fine fractions by sedimentation with theresults shown in columns four and five-of Ta ble II. It will be seenthat the fine fraction amounted to 53. 5%;of the weight of the dry feedto the original fiotation operation, and contained particles ofkaolinwhich-were 97% finer than 5 microns and 83% -'-fin er' than2'microns.

The coarse fractionfrom the product of the primary grinding run was putthroughthe heated pressure tube apparatus again under conditions-similarto those described for the first run. "The yields and particlesize 1distribution analysis-of*-the--screen product and its sedimentedfractions are -show-n'in columns six, seven, and eight of Table II. Itis apparent from the results in column eight that rerunningthe coarsefraction through the grinding step-increased the yield of the finefraction.

Table III, illustrates the-outstanding brightness of the clay which isachievedby the-;practice -0f my invention. All of the weights thereinare'reported on a volatilefree basis, volatile-free weight being theweight of the clay after heating said clay'yat ;a-temperature of aboutY1eld, percent weight.

1700 F. to essentially constant weight. Brightness is tested by TAPPIStandard MethodT-64'6 as described Table ll .PARTICLE SIZEDISTRIBUTIONANALYSIS OF FINE GRIN-DINGQOPE'RATION Rernn 0f Coarse Frac-Percent FinerThantion from Primary Run Fractionated Product FractionatedProduct -Micr0ns PrimaryRun Prod- Based on 100% dry feed to theflotationbperation.

on pages 159A and 160A;of the October 1954 issue of Tappi (a monthlypublicatiomof theTechnical Association of Pulp and Paper Industry). Themethod measures the light reflectance of a clay sample and thus givesquantitative indication of its brightness or whiteness.

. V V 7 In the tables thebrightness of the beneficiated clay andbeneficiated disintegrated clay are reported below as bleached andunbleached materials, bleaching being accomplished with a conventionalzinc hydrosulfite bleaching liquor. The brightness of these samplesappear below compared to an average brightness, determined in the samefashion, of a kaolin of equivalent particle size distribution preparedby the conventional method of water-washing, fractionation andbleaching.

The above table demonstrates that my method of treating kaolin resultsin a brightness markedly superior to that achieved by either theconventional bleaching method for brightening clay or by flotationwithout subsequent disintegration of the beneficiated clay. The tablealso shows that when my flotation-disintegration treatment is followedby bleaching the resultant product has a brightness unmatched by methodsheretofore known.

It will be understood that the invention is suscep tible to numerousvariations without departing from th spirit and scope of the invention.

I claim:

1. A method for improving the brightness of coarse kaolin claycomprising dispersing said clay in an aqueous medium to form an aqueouspulp of said clay, subjecting said aqueous pulp to negative-ion frothflotation treatment with a negative-ion reagent to thereby produce afroth-product material which is a concentrate of the colored impuritiesin the original clay and a machine discharge slurry including kaolinclay of improved brightness, forming said slurry into a flowing confinedstream, vaporizing at least a portion of the liquid component of saidstream to form a dispersion of clay particles in the resulting vapor andsubjecting the dispersion to turbulence and high velocity whereby saidparticles impinge against one another and disintegrate to relativelyfine particles of enhanced brightness.

2. The method of claim 1 wherein said negative-ion reagent comprises afatty acid.

3. A method for improving the brightness of coarse kaolin claycomprising dispersing said clay in an aqueous medium to form an aqueouspulp of said clay, subjecting said aqueous pulp to negative-ion frothflotation treatment with a negative-ion reagent to thereby produce afroth-product material which is a concentrate of the colored impuritiesin the original clay and a machine discharge slurry of clay product,adjusting the solids con tent of said machine discharge slurry to about15 to 40 percent by weight, forcing said slurry into and through anelongated zone of restricted cross-section, heating said slurry duringpassage through said zone to a temperature above the boiling point ofthe liquid component of said slurry at the pressure existing in saidtube, vaporizing at least a portion of said liquid component duringpassage through said tube to form a dispersion of clay in the resultantvapor, subjecting the said dispersion to turbulence and highvvelocitywhereby said particles impinge against one another and disintegrate torelatively fine particles of enhanced brightness.

4. The method of claim 3 including the additional steps of dischargingthe relatively fine particles into a stream of water to form a secondslurry and subjecting said second slurry to hydraulic classification.

5. The method of claim 3 in which the flotation is carried out in thepresence of at least one auxiliary agent selected from the groupconsisting of ammonium sulfate, magnesium sulfate and potassium sulfate.

6. The method of claim 3 wherein the solids content of said machinedischarge slurry is adjusted with water.

7. The method of claim 3 wherein said negative-ion reagent comprises afatty acid.

8.A method for improving the brightness of coarse kaolin clay comprisingdispersing said clay in an aqueous medium to form an aqueous pulp ofsaid clay, subjecting said aqueous pulp to negative-ion froth flotationin an aqueous pulp with a negative-ion reagent to thereby produce afroth-product material which is a concentrate of the color impurities inthe original kaolin and a machine discharge kaolin product andsimultaneously increasing the fineness and further enhancing thebrightness of said machine discharge kaolin by forming a slurry of saidmachine discharge kaolin in a vaporizable liquid, passing said slurryinto an initial portion of an elongated fluid conduit, heating saidslurry in said fluid conduit to vaporize liquid and form a dispersion ofsaid clay in hot vapor, passing said dispersion through a latter portionof said fluid conduit at high velocity in turbulent flow to 'causeparticles of said clay to impinge against one another and disintegrateto relatively fine particles.

9. The method of claim 8 in which said coarse kaolin clay consistsessentially of particles less than about 44 microns and greater thanabout 2 microns.

10. The method of claim 8 in which said coarse kaolin clay comprisesparticles within the range of from about 2 to 10 microns.

11. The method of claim 8 wherein said negative-ion reagent comprises afatty acid.

12. A method for improving the brightness of kaolin clay comprisingdispersing kaolin clay consisting essentially of particles finer thanabout 44 microns and coarser than about 2 microns in an aqueous mediumto form an aqueous pulp of said clay, subjecting said aqueous pulp tofroth flotation in an aqueous pulp with a negative-ion collector reagentselective to the colored impurities in the original kaolin clay tothereby produce a froth-product material which is a concentrate of thecolored impurities in the original kaolin clay and a machine dischargekaolin clay product of enhanced brightness and, thereafter,simultaneously increasing the fineness and further enhancing thebrightness of said machine discharge kaolin clay by forming an aqueousslurry of said machine discharge kaolin clay, passing said aqueousslurry into an initial portion of an elongated fluid conduit, heatingsaid slurry in said conduit to vaporize water and form a dispersion ofsaid kaolin clay in water vapor and passing said dispersion through alatter portion of said conduit at high velocity in turbulent flow tocause particles of said clay to impinge against one another anddisintegrate to relatively fine particles.

13. The method of claim 12 including the additional steps of dischargingsaid relatively fine particles from said latter portion of said condiutinto a stream of water to form a second slurry and subjecting saidsecond slurry to hydraulic classification.

References Cited in the file of this patent UNITED STATES PATENTSEastman et al Feb. 21,

1. A METHOD FOR IMPROVING THE BRIGHTNESS OF COARSE KAOLIN CLAYCOMPRISING DISPERSING SAID CLAY IN AN AQUEOUS MEDIUM TO FORM AN AQUEOUSPULP OF SAID CLAY, SUBJECTING SAID AQUEOUS PULP TO NEGATIVE-ION FROTHFLOTATION TREATMENT WITH A NEGATIVE-ION REAGENT TO THEREBY PRODUCE AFORTH-PRODUCT MATERIAL WHICH IS A CONCENTRATE OF THE COLORED IMPURITIESIN THE ORIGINAL CLAY AND A MACHINE DISCHARGE SLURRY INCLUDING KAOLINCLAY OF IMPROVED BRIGHTNESS, FORMING SAID SLURRY INTO A FLOWING CONFINEDSTREAM, VAPORIZING AT LEAST A PORTION OF THE LIQUID COMPONENT OF SAIDSTREAM TO FORM A DISPERSION OF CLAY PARTICLES IN THE RESULTING VAPOR ANDSUBJECTING THE DISPERSION TO TURBULENCE AND HIGH VELOCITY WHEREBY SAIDPARTICLES IMPINGE AGAINST ONE ANOTHER AND DISINTEGRATE TO RELATIVELYFINE PARTICLES OF ENHANCED BRIGHTNESS.